1. Field of the Invention
The present invention relates to a process for producing a perfluoro copolymer having a cyclic structure in its main chain.
2. Discussion of Background
With respect to a fluorine-containing polymer having a cyclic structure in its main chain, particularly a perfluoro cyclic polymer, Japanese Unexamined Patent Publication No. 13125/1989 discloses that CF.sub.2 =CFO(CF.sub.2).sub.n CF=CF.sub.2 undergoes cyclopolymerization by itself to give a non-crystalline solvent-soluble polymer having a cyclic structure of the formula: ##STR1## wherein n is 1 or 2, and/or the formula: ##STR2## wherein n is 1 or 2. Usually, the softening point of a non-crystalline polymer is governed by the glass transition temperature (Tg). Therefore, many physical properties such as mechanical properties and electrical properties undergo substantial changes around this glass transition temperature. The cyclic polymer obtained by the homopolymerization of CF.sub.2 =CFO(CF.sub.2).sub.n CF=CF.sub.2 is no exception, and when n=1, Tg of the polymer=69.degree. C., and even when n=2, Tg=108.degree. C. Thus, the thermal deformation temperature is not so high that there has been a problem that the polymer is not suitable for use at a high temperature.
On the other hand, with respect to a perfluoro cyclic polymer, U.S. Pat. No. 4,754,009 discloses that an amorphous polymer can be obtained by the copolymerization of perfluoro-2,2-dimethyl-1,3-dioxol (hereinafter referred to simply as PDD) of the formula: ##STR3## and tetrafluoroethylene, and this copolymer is soluble in a solvent and excellent in the light transmittance and has a very high Tg e.g. at a level of 250.degree. C. when the content of PDD is 89 mol %.
However, such a copolymer has a problem that with an increase of the tetrafluoroethylene content, Tg substantially decreases, and the crystallinity appears, whereby the transparency deteriorates, and the solvent-solubility tends to be impaired. Besides, this copolymer is obtained by a reaction of a gaseous monomer and a liquid monomer and thus has problems such that it is difficult to control the composition, it is difficult to obtain a copolymer having a desired Tg, and a homopolymer of tetrafluoroethylene is likely to be formed as a byproduct.